The invention relates to a device for recognizing a child's seat which is strapped to the front passenger's seat of a motor vehicle.
In the case of motor vehicles having a built-in front seat passenger's side airbag, the degree of injury to the front seat passenger in the event of a frontal impact is reduced in that the airbag is triggered and is changed into a state in which it provides a protective effect. However, if the front passenger's seat is occupied by a child in a child's seat, airbag release is undesirable since an optimum protective effect of the airbag is not ensured and an additional risk of injury by the airbag cannot be precluded for all types of child seats. This applies, in particular, for child's seats (reboard seats) which face the rear of the vehicle.
A resistive membrane pressure sensor, which is constructed as a sensor mat for seat occupancy detection is known from German Patent document DE 42 37 072 C1. Since such a membrane pressure sensor emits an analog signal which depends on the normal force on its surface, the sensor mat cannot only be used to recognize whether or not the seat is occupied, but a coarse interrogation of the weight resting on the seat can also be carried out. A lower limit value of, for example, 12 kg can then be set, beneath which airbag release is suppressed. This is based on the assumption that, if the weight is less than this, the seat is either unoccupied or is occupied by a child in a child's seat. This method is very imprecise, since a consistent definition of the lower limit value is impossible: this is because both the weight of the child and the weight of the child's seat are subject to a wide band spread. In addition, the various seat types differ greatly from one another in the local load on the seat surface, which can corrupt the weight measurement.
There is therefore needed a technical solution by means of which the presence of a child's seat on the front passenger's seat can be detected in a simple and reliable manner.
This need is met according to the present invention by a device for recognizing a child's seat which is strapped to the front passenger's seat of a motor vehicle. A base station in the interior of the vehicle emits an electromagnetic measurement field. An identification carrier, by means of which the measurement field is physically changed in a characteristic manner, is fitted to the child's seat. A change in the measurement field can be recorded by the base station. When a change in the measurement field occurs which is characteristic of an identification carrier moved into the measurement field, the base station emits a corresponding identification signal.
The device according to the present invention has the advantage that the child's seat recognition is not based on a weight measurement and is thus independent of the weight of the child in the child's seat. The child's seat recognition according to the invention is dependent only on the child's seat being provided with an identification carrier. This can either already been integrated in the child's seat in the factory, or can be subsequently fitted as a cost-effective retrofitted part without any problems, for example by bonding or clipping on. Since the identification carrier is recognized without making contact by means of a measurement field, on the one hand no wear occurs and on the other hand handling of the child's seat is not adversely affected.
A suitable principle for physical marking by means of an identification carrier is known per se and is used, for example, in warehouses for marking goods which have not yet been purchased by a buyer. In this case, the identification carrier in the simplest case comprises a tuned circuit, which is made up of a coil and a capacitor, and attenuates a radio-frequency electromagnetic field, which is emitted from a base station, in a characteristic manner.
Another type of identification carrier is known, for example, from manufacturing technology and is described in the article by S. Drews and W. P. Schmidt, "Datentrager werden intelligent" (Data carriers become intelligent) in me, Volume 6 (1992), Issue 2. Intelligent, programmable data carriers are now used in manufacturing. The carriers are fitted directly on the workpiece and are used as information which accompanies the material. The data carriers can be read without making contact. They are supplied inductively from the base station and thus do not require their own electrical power supply either for reading or programming, or for maintaining the data. Such a data carrier can be implemented cost-effectively and essentially comprises a coil, possibly a capacitor for tuning, and an integrated circuit chip. The base station produces a constant electromagnetic alternating field in the frequency range from 100 kHz-50 MHz. If the data carrier comes into the range of the transmitting coil, then it obtains its energy by transformer coupling and starts to read its stored data and to transmit the data to the base station. The transmission is based on a method which is known from transponders and in the case of which the electromagnetic alternating field is amplitude-modulated by different attenuation by the identification carrier. The base station evaluates the modulation.
One advantageous design is for the transmitting coil to be integrated in a seat occupancy sensor which is constructed as a sensor mat.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.